Question substituted by setter in permitted numbers of letters (5)
I believe the answer is:
iotas
'letters' is the definition.
(iota is a kind of letter)
'question substituted by setter in permitted numbers of' is the wordplay.
I cannot quite understand how this works, but
'setter' could be 'i' (the person that set the crossword) and 'i' is located in the answer.
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'tas' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for iotas that I've seen before include "Jots, very small amounts" , "Tiny scraps" , "Tiny amounts" .)